The effects of harsh radiation environments, such as are encountered in space, military, or nuclear power applications, are known to have deleterious effects on semiconductor devices and circuits. Recent studies have further shown that prolonged exposures to even low levels of radiation can degrade the performance of electronic devices. Compounding these effects, the continued reduction in feature size of modern commercial integrated circuits (ICs) can result in these devices becoming even more sensitive to transient ionizing radiation effects. Ionizing radiation produces electron-hole pairs (i.e., electrical charge) in semiconductor material that may be collected by potentially sensitive circuit nodes. The generation and collection of this unintended electrical charge within the circuit can alter the performance and reliability of specific circuit structures within the device.
It is particularly desirable to be able to measure and monitor changes in the electrical charge collection within a device to detect changes in its radiation response with age. Age-related changes in the radiation response of numerous types of semiconductor devices have been reported by several groups in recent years. See A. P. Karmarkar et al., “Aging and Baking Effects on the Radiation Hardness of MOS Capacitors,” IEEE Trans. Nucl. Sci. 48(6), 2158 (2001); and V. S. Pershenkov et al., “Effect of Aging on Radiation Response of Bipolar Transistors,” IEEE Trans. Nucl. Sci. 48(6), 2164 (2001). Studies of aging in ICs suggest that achieving a detailed understanding of some of the natural changes that occur in these devices over long storage times requires detailed, repeated and non-destructive examination of the functional and parametric properties of the dose-rate response of semiconductor devices.
Historically, the greatest obstacle to measure the radiation response of electronic devices used in long-lived electrical systems has been the destructive nature of most traditional radiation testing techniques. The accumulation of total dose damage with each subsequent re-test of a device can change its radiation response, (e.g. dose-rate, single event upset, single event latch-up, or burn-out) independent of any other hidden aging effects. Therefore, a need remains for a non-destructive method to measure the dose-rate response of semiconductor devices, particularly as they age.
In most modern semiconductor devices, it is only the charge produced in the top few tens of microns of silicon that can be collected by the electrically active regions of the device; electron-hole pairs produced deeper in the silicon (i.e., in the heavily doped silicon beneath the epitaxial layer) recombine before reaching sensitive circuit nodes. Thus, while it is true that the exposure of a silicon device to severe gamma- or x-ray fluxes gives rise to electron-hole pair production throughout the entire silicon die (and package and circuit board and the entire system), the vast majority of this electrical charge has no path to directly affect the internal electrical operation of the IC. If the charge is not produced within a diffusion length of the electrically active regions of the circuit, it will merely recombine, or be trapped, without perturbing the circuit.
According to the present invention, an infrared laser of tens of watts can produce as many electron-hole pairs in the electrically active regions of a semiconductor device as are produced during severe radiation exposures of MeV gamma-rays or keV x-rays. The apparently huge difference in power between the laser and the gamma- or x-ray exposure conditions is reconciled not when viewed in terms of how much energy the device is exposed to, but rather how much of that energy is actually deposited in the device where it can affect the device's electrical operation. When viewed in this context, very few of the incident gamma- or x-rays actually produce electron-hole pairs within the device and even fewer of these electron-hole pairs are produced within the collection volume of the electrically active regions of the device. Therefore, infrared laser-based irradiation testing can produce dose-rate response in semiconductor devices that replicates the response to much more penetrating ionizing radiations.
Furthermore, laser-based probing can detect aging effects without the damage associated with particle beam or x-ray exposure techniques. Thus, dose-rate failure testing of ICs can be performed repeatedly, non-destructively, and inexpensively on a bench top. It is important to note that broad beams of energetic x-rays or electrons provide higher fidelity simulations of actual high-energy radiation environments than the infrared laser. Indeed, such exposures reproduce many bulk material radiation effects (e.g., shock, thermal effects, and secondary radiations) that the laser cannot. However, in the context of photocurrent collection and circuit electrical operation, the additional energy deposition deep in the sample only produces electron-hole pairs that may contribute to thermal and mechanical effects, but not to electrical behavior.